The present invention relates to fuel cell systems, and more particularly to a fuel cell stack and method to operate a fuel cell stack.
Fuel cells have been used as a power source in many applications, for example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell, and oxygen is supplied as the oxidant to the cathode. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued Dec. 21, 1993 and May 31, 1994 respectively, and commonly assigned to General Motors Corporation. MEAs include a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and a cathode catalyst on the opposite face.
The term xe2x80x9cfuel cellxe2x80x9d is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack. Each cell within the stack comprises a membrane electrode assembly which provides its increment of voltage. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113 assigned to General Motors Corporation.
The electrically conductive elements sandwiching the MEAs may contain an array of channels or grooves in the faces thereof for distributing the fuel cells gaseous reactants over the surfaces of the respective cathode and anode. In the fuel cell stack, a plurality of cells are stacked together in electrical series while being separated one from the next by a gas impermeable, electrically conductive bipolar plate. In a common, single pass-through design, the reactants are supplied to the fuel cells through individual inlet manifolds and headers. From an inlet manifold providing flow to an inlet header, the reactant, for instance the anode flow, is divided in a number of flow paths feeding individual cells. All of the reactant (in this case the anode flow) as exhaust gas leaves the individual cells, mixes in the outlet header and exits the stack through the outlet manifold. In the single pass-through design, the anode sides of all cells have the same inlet hydrogen concentration.
The disadvantage of the single pass-through design of directing reactant gas is that the fuel cell stack is unable to stably operate at low stoichiometry; that is, near the mass flow of reactants needed to satisfy a given power output. It is therefore difficult to achieve efficient hydrogen or oxygen utilization. As a result, system efficiency is not optimized.
Stack designs which partially correct the above situation are known, such as the stack design of U.S. Pat. No. 5,478,662 issued to Strasser. In stacks such as the Strasser design, individual groups of parallel cells are arranged wherein the flow within each cell of each group is in parallel, and all the flow from each group flows between groups in series. In one exemplary stack design, each stack group has an inlet and an outlet manifold and each outlet manifold has drains to collect water which is formed as a reaction product. If not drained, the water builds up in succeeding cell groups. The number of individual fuel cells normally varies in these stack designs wherein the initial or upstream segments of cells contain the largest number of individual fuel cells and each successive segment provides a reduced quantity of fuel cells. With this type of configuration the last segment of the set of segments normally has the fewest number of individual fuel cells.
A disadvantage of common grouped stack designs results when reactant gases as reformate have inert components flowing through the stack. The inert gas portion of the reformate fuel which is retained through each group of the stack concentrates as the quantity of fuel cells in each segment decreases and can result in the final stack segment controlling the overall pressure drop through the stack. Normally, a hydrogen reformate stream entering a fuel cell stack comprises about 40% by volume of hydrogen. The remaining volume comprises nitrogen and other gases. Only the hydrogen is consumed (forming water) by the fuel cell stack, therefore 60% of the anode gas volume as inert gas flows through each group of the stack. This volume of inert gas determines the pressure drop across the smallest groups of the stack. When air is used as the cathode gas, oxygen concentration normally comprises between 20% to 40% by volume of the flow. The remaining volume of about 60% to 80% of the cathode flow comprises nitrogen with other inert gases.
The above series/parallel stack designs normally provide a serpentine type flow pattern throughout the stack. A serpentine flow path results in both anode and cathode side reactant flows which are either horizontal throughout the stack, or that must overcome gravity for one or more individual segments. Water build-up in the fuel cells inhibits reactant contact with the catalyst materials of the fuel cells, thus decreasing stack efficiency. Water generated in the non-gravity assisted groups must be forced through the cells with the reactant gas or permitted to xe2x80x9cback flowxe2x80x9d against the reactant flow direction for removal from the stack, reducing efficiency of the stack.
The above drawbacks for fuel cell stacks are overcome by a stack design of the present invention. The stack design of the present invention incorporates individual segments of fuel cell elements arranged in equivalent or different quantities of fuel cell elements within each segment. Each segment is arranged to provide flow within each fuel cell in a gravity assisted direction. This arrangement increases the stability of the fuel cell stack operation by allowing the entire volume of either the anode side or cathode side, or both flows, to be distributed through segments of the stack wherein only a portion of the total number of the cells are present.
In a preferred embodiment, a first segment normally having a greatest percentage of stack fuel cells depletes the greatest percentage of the reactant entering that segment. The first segment is arranged to provide downward or gravity assisted flow through each cell. The reactant exiting the first segment is directed to a second segment having a smaller quantity of fuel cells disposed therein, also arranged to provide a gravity assisted flow. This second segment reacts the majority of remaining fuel from the reactant flow. If necessary, a third or more segments of cells are employed to maximize utilization of the hydrogen and oxygen from the reactant flows. Each segment provides fuel cells in parallel and is arranged for gravity assisted flow through each cell of the segment. Overall stoichiometry of the stack is improved by the design of the present invention.
A separator segment is disposed between each stack segment wherein all of the flow exiting the preceding segment is routed through the separator segment. In a preferred embodiment, the separator segments are disposed between a pair of bipolar plates lacking a membrane electrode assembly (MEA). The purpose of the separator segment is to redirect all the flow from the outlet or lower portion of that segment into the inlet or upper portion of the next succeeding segment without any reaction in the fuel flow and therefore generating no additional water. Each separator segment is a structural member adjoining individual segments of fuel cell elements. A separate separator segment is provided between each two segments of fuel cells to separately redirect flow between individual flow groups, i.e., anode, cathode and coolant. The flow exiting each flow segment is collected at a lower section of that stack segment and redirected generally upwardly to a top inlet of the next succeeding segment to form a xe2x80x9ccascadedxe2x80x9d stack design. This provides flow through the individual fuel cell segments in only a gravity assisted direction, i.e., downward.
Water which forms as a reaction product in each individual segment is collectively drained at the base of each following separator segment. This water volume is discharged through drain lines to a discharge point in the stack. This ensures that the water formed in each segment of the stack drains in a gravity assisted direction toward the gravity drain points at the base of each separator segment and provides an efficient removal method for liquid water generated in the stack.
A further aspect of the present invention also provides selected use of modified flow stream geometry for fuel cell elements within each fuel cell segment. By modifying the flow path geometry, i.e., increasing flow passage size or length of fuel cell elements within at least one selected segment, different reformate streams of reactant having different volumetric percentages of inert gas(es) can be employed. The reformate fuel stream normally comprises a higher inert gas volume compared to the volume of the reactant gas i.e., hydrogen. As an example, a desirable reformate fuel stream initially comprises about 40% hydrogen by volume and about 60% inert gas by volume. As the stream traverses the stack and hydrogen is reacted to form water, the water drains off leaving an increasing volume of inert gas in the stream. It is therefore desirable to decrease the number of fuel cells in successive segments based on decreasing hydrogen volume. Decreasing fuel cell quantity while retaining fuel cell size can result in increased pressure drop in the downstream segment, thus controlling or limiting fuel stream flow through the stack. By increasing fuel cell cross-section in selected downstream segment(s), pressure drop does not increase as the number of cells decreases. By decreasing fuel cell length in select downstream segment(s), pressure drop does not increase as the number of cells decreases. The result is increased efficiency at reduced cost because fewer fuel cells are used while the net flow rate is retained. The inert gas which is not reacted by the fuel cell stack therefore does not control the overall pressure drop across the stack.
In a further aspect of the present invention, series flow of coolant through the individual segments is provided. The coolant enters the end of the stack where the reactants enter the stack and follows the fuel stream flow. The advantage of series flow is that coolant at its lowest temperature enters the stack at the point where the driest reactant gas enters the stack, providing the maximum reactant gas temperature drop and therefore the highest relative humidity for the inlet gas. Providing the lowest temperature for the entering reactant gas reduces its dew point and requires very little water vapor production to achieve the desired 100% relative humidity for fuel stream flow through the stack. It is normally desirable to provide about 100% relative humidity (RH) in each segment of the stack because water is continuously being generated and excess liquid water hinders flow and hydrogen contact with the fuel cells.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.